move hardbeat handling into backend, add publish lanestate for lighttower

src/master.h

@@ -34,6 +34,8 @@ struct ButtonConfig {
   uint8_t mac[6];
   bool isAssigned = false;
   float voltage = 0.0;
+  unsigned long lastHeartbeat = 0; // Zeit des letzten Heartbeats (millis)
+  bool heartbeatActive = false;    // Status: aktiv/inaktiv
 };
 
 struct ButtonConfigs {